Friday, May 6, 2016

Lake Toba

70,000 years ago the volcano at Lake Toba
in Sumatra, Indonesia--The Ring of Fire-- erupted. It is believed to be
one of the most extraordinary events in human history. It was 100
times greater than that of the largest volcanic eruption in recent
history, the 1815 eruption of Mount Tambora in Indonesia, which caused
the 1816 "Year Without a Summer" in the northern hemisphere. The Toba eruption apparently coincided with the onset of the last glacial period.
Michael L. Rampino and Stephen Self argue that the eruption caused a
"brief, dramatic cooling or 'volcanic winter'", which resulted in a drop
of the global mean surface temperature by 3–5 °C and accelerated the
transition from warm to cold temperatures of the last glacial cycle. The
eruption may have caused this 1,000-year period of cooler temperatures
(stadial). Assuming an emission of six billion tons of sulphur dioxide,
computer simulations concluded that a maximum global cooling of
approximately 15 °C occurred for three years after the eruption, and
that this cooling would last for decades, devastating life.

This
is connected to the genetic "Bottleneck Theory." According to the
bottleneck theory, between 50,000 and 100,000 years ago human
populations sharply decreased to 3,000–10,000 surviving individuals.
This is supported by genetic evidence suggesting that today's humans are
descended from a very small population of between 1,000 and 10,000
breeding pairs that existed about 70,000 years ago. The theory goes that
the Toba disaster altered climate, animal populations and plant life
and may have been a factor in the decrease in human numbers.

Recent research shows the extent of climate change was much smaller than believed by proponents of the theory. In addition, coalescence times for Y-chromosomal and mitochondrial DNA have been revised to well above 100,000 years since 2011.
In 2000, a Molecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change.This would be consistent with suggestions that in sub-Saharan Africa numbers could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in the Late Stone Age.

(Genetic drift can cause big losses of genetic variation for small populations. Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted. This thesis of Toba's impact has a lot of opponents who believe that the bottleneck period did occur, it just occurred hundred of thousands of years earlier.)